Green sulfur bacteria are obligatory anaerobic phototrophs that grow only by photosynthesis. Therefore, they have superior photosynthetic capability, and characteristically contain chlorosome which is an extramembranous antenna system adapted to an extremely weak light. The light-harvesting part in the chlorosome is formed from self-aggregates consisting only of pigments free of the involvement of protein (non-patent document 1). This is the sole exception since pigments perform function in a protein support in the antenna system of all other photosynthetic organisms. Chlorosome can be isolated and purified easily from the living organism, and can be reconstituted after being decomposed outside the living organism. Furthermore, when a pigment that induces a photochemical reaction is added during reconstitution, functional aggregates can be created easily (non-patent document 2).
As self-aggregating pigments in the chlorosome, bacteriochlorophyll (BChl) c, d and e are known (FIG. 1), and which pigment is employed varies depending on the species of the green sulfur bacteria. BChl f and a green sulfur bacterium having BChl f have not been found in the natural world. Previous reports have confirmed that artificial genetic engineering can be performed in the green sulfur bacterium Chlorobaculum tepidum having BChl c and the green sulfur bacterium Chlorobaculum parvum having BChl d (non-patent documents 3, 4). Therefore, studies relating to the biosynthetic pathway of the pigment have progressed and all synthase genes have been elucidated. By modifying the elucidated enzyme genes, chlorosomes constituted by various pigment molecular species have been produced in the living organism of variants. There are also variants greatly contributing to the elucidation of the structure of chlorosomes (non-patent document 5).